Various networks having redundancy properties are already disclosed in EP 1 260 081 B1. Inter alia a so-called Optical Switch Module (OSM) having a plurality of ports for the attachment of further network components is described there. In order to construct an Ethernet or Fast Ethernet network, e.g. terminals are attached to the ports and are therefore connected to each other via the OSM. A plurality of OSMs can be interconnected in a continuous line structure to form a string. An optical ring is produced if the two line ends are attached to an Optical Redundancy Manager (ORM). Test messages which the ORM inputs into the two line ends are used to check whether there is an interruption in the string. If there is no error, i.e. if the string is not interrupted, the ORM separates the two line ends and if there is an error, the two line ends are connected together by the ORM. In a network comprising a plurality of redundant optical rings, two optical rings are coupled together via two OSMs. The two OSMs which are connected between the optical rings form a communication channel in each case, of which one is active and one is switched to a standby mode. The two OSMs exchange their status via special standby ports which are connected to each other by means of cables. If the connection via the active OSM fails, the redundant OSM enables its communication channel, i.e. it switches from standby mode to active mode. A comparatively short reconfiguration time is achieved in this way. This prevents physical errors, temporary electromagnetic interference, network upgrades or component replacements from impairing the communication between the network components for an unacceptably long time. The disadvantage in this case is that two additional devices are required for the redundant connection of two optical rings, and that these devices must additionally be connected together. In order to avoid this disadvantage, the above cited patent document proposes that a switch module which is located at the active communication channel cyclically sends so-called port-select messages to the other switch modules that are arranged in the same string, while the communication channel is uninterrupted. The switch module which is located in standby mode at the other communication channel monitors the first communication channel by analyzing the port-select messages. Although a comparatively short reconfiguration time is likewise achieved in the known network, the configuration of the switch modules in the startup phase of the network is disadvantageously carried out using a resource-intensive algorithm and requires more time due to the multiplicity of messages that must be sent.
Further networks having redundancy management are disclosed in the publications EP-A-0 403 763 and EP-A-0 560 122.
The standardized Spanning Tree and Rapid Spanning Tree protocols likewise allow redundancy management in an Ethernet network. However, these methods cannot always be used in industrial communications due to the long reconfiguration times, particularly if time intervals of several seconds during which the data communication is disrupted are not acceptable.